To lighten the weight of automobiles, an issue arising from the problem of global warming, it may be preferable to make steel sheet used for automobiles that is very high in strength. When making a high-strength steel sheet, elongation and r values may decrease and formability may be reduced. To solve this problem, techniques for hot forming materials and using heat to increase strength are described, e.g., in Japanese Patent Publication (A) No. 2000-234153. This publication describes a technique which includes controlling a steel composition, heating the steel in a ferrite temperature region, and utilizing precipitation strengthening in this temperature region to increase the strength.
Japanese Patent Publication (A) No. 2000-87183 describes a high-strength steel sheet which can provide improved precision for press forming by reducing the yield strength at a formation temperature to a lower value than the yield strength at ordinary temperatures. However, the strength that can be obtained using such techniques may be limited. Alternatively, Japanese Patent Publication (A) No. 2000-38640 describes a technique for obtaining a higher strength by heating a material to a high-temperature austenite single-phase region after formation, and transforming it to a hard phase in a subsequent cooling process.
Heating and rapidly cooling a sheet after it is formed can decrease precision of the formed shape. To avoid this problem, techniques for heating steel sheet to an austenite single-phase region, then cooling the sheet in a press formation procedure using a cooling rate of at least the critical cooling rate of martensite transformation, as determined by the steel compositions, are described, e.g., in Cornette et al., “High Strength Steels for Automotive Safety Parts,” (Paper No. SAE, 2001-01-0078, SAE World Congress, 2001) and in Japanese Patent Publication (A) No. 2001-181833. The Cornette publication describes a technique which can provide suppression of scaling of the surface at a time of heating by using Al-plated steel sheet. This type of pressing procedure can be referred to as “hot pressing.”
Japanese Patent Publication (A) No. 2003-147499 describes a technique for using steel sheet covered by a plating layer that includes an Fe—Zn alloy for hot pressing, while Japanese Patent Publication (A) No. 2003-41343 describes a technique for using Al-based plated steel sheet covered by a plating layer that includes an Fe—Al alloy for hot pressing. Also, Japanese Patent Publication (A) No. 2002-282951 describes an exemplary technique using a die and punch to press a heated metal sheet, where the die clearance can be determined based on formability and hardenability considerations.
Thus, high strength steel sheet which may be used for automobiles, etc., may exhibit problems with respect to low formability and/or hydrogen embrittlement (which may be referred to as aging cracks or delayed fracture), particularly in high-strength materials of over 1000 MPa. Therefore, there may be a need for improved steel sheet which may be used for hot pressing, and it may further be desirable to decrease the amount of hydrogen in the material.